In a wireless multiple access communication system, wireless traffic channel resources, e.g., bandwidth and time interval, are shared by all the wireless terminals, e.g., mobile units, in a particular cell. Efficient allocation of the traffic channel resources is very important, as it may directly impact utilization of the traffic channel resources, and Quality of Services (QoS) perceived by individual wireless terminal users. One such wireless communication system is the Orthogonal Frequency Division Multiplexing (OFDM) based Multiple Access system.
OFDM, as a combination of modulation and multiple access schemes, segments a communication channel for many users to share. Whereas Time Division Multiple Access (TDMA) segments according to time and Code Division Multiple Access (CDMA) segments according to spreading codes, OFDM segments according to frequency. OFDM is a technique that divides the spectrum into a number of equally spaced tones, and carries a portion of a user's information on each tone. Each of these equally spaced tones is also termed as a frequency subcarrier, because OFDM may be viewed as a form of frequency division multiplexing (FDM). Specifically, OFDM has an important property that each tone is orthogonal with every other tone and allows the spectrum of each tone to overlap. In addition, since these tones are orthogonal, they do not interfere with each other. By allowing the tones to overlap, the total amount of spectrum required is reduced.
OFDM may also be considered as a multiple access technique since an individual tone or groups of tones may be assigned to different users. Multiple users share a given bandwidth in this manner, yielding a system called orthogonal Frequency Division Multiple Access, or OFDMA. Each user may be assigned a predetermined number of tones when they have information to send, or alternatively, a user may be assigned a variable number of tones based on the amount of information the user has to send. The assignments are controlled by a Media Access Control (MAC) layer, which schedules resource assignments based on user demand.
OFDMA is a system in which a plurality of users may perform multiple access using OFDM. In a conventional OFDMA data transmission apparatus, a method is proposed to carry out both frequency division and time division when multiple access is performed. Also, to improve error correction capabilities, diversity is carried out in a frequency direction and in a time direction.
The recently defined 802.16 standard uses OFDM for physical layer transmissions, because OFDM has been shown to be robust against frequency selective fading. OFDM-based networks may provide users with high data rates with a much wider coverage than WiFi networks (802.11). OFDM-based networks may also support QoS services, and 802.16e may provide support for limited mobility. In OFDM networks, resources are scheduled in frequency, time and power domains. In the downlink, the rate achieved by a user increases with the number of subcarriers (frequency domain) assigned, the number of OFDMA symbols (time domain) assigned, and the fraction of Base Station (BS) power allocated. The same is true for the uplink, except that there is a limit on the total transmission power of each Subscriber Station (SS). Both uplink and downlink resource allocation are performed by a BS.
The downlink scheduling problem has been addressed recently. Conventional methods focus on maximizing the sum rate capacity with full buffer traffic. A utility-based approach is used to solve frequency dependent scheduling cases. Another approach focuses on distributed subcarrier channels. However, in each of these cases only a single type of user was considered. Therefore, what is needed is a novel approach that considers the case of mixed user types, and focuses on the problem of optimally dividing resources between users requiring frequency diverse channels and users requiring frequency dependent scheduling.